As generally depicted in FIG. 1, optical wavelength multiplexing and demultiplexing have been accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides 2 communicating with the input of a star coupler 4. The output of the star coupler 4 communicates with an optical grating 6 comprising a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbor by a predetermined amount. The grating 6 is connected to the input of a second star coupler 8, the outputs 9 of which form the outputs of the switching, multiplexing, and demultiplexing apparatus. Examples of such interconnection apparatuses are disclosed in U.S. Patents 5,002,350 and 5,136,671, which are expressly incorporated by reference herein.
The overall design, and particularly the geometry, of such an interconnection apparatus may be such that a plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. A similar apparatus may also perform a demultiplexing function. In this situation, an input wavelength is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelength also permits switching between any selected input port to any selected output port. Accordingly, these devices are generally referred to as frequency routing devices and more specifically wavelength division multiplexers (WDM).
Ideally, the individual wavelength-channel positions, as measured by the center point of the passband, of the WDMs and the associated transmitter(s) should be aligned to a predefined wavelength grid, referenced herein as .lambda..sub.0, .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, . . . , .lambda..sub.n. Unfortunately however, in practice, the wavelengths of both the transmitter(s) and WDM channels drift with time and/or have initial fabrication errors. Such drifting or fabrication errors each result in the wavelengths of the respective optical devices to not be aligned as desired and thus adversely affect the operation of that device within a communication system. At present, either fabrication error and/or variances during operation can cause WDM components themselves to often exhibit about a 0.1 nm or 10 GHz shift or tolarance within a 100 GHz system while a transmitter may often exhibit about twice that amount of offset or about 20 GHz in a 100 GHz system. Furthermore, to be effectively used in the increasingly demanding optical communication systems of today where WDM systems are going to smaller channel spacings, i.e., less than about 50 GHz, and large channel counts, i.e. greater than or equal to about 32 channels, improvement is needed in the ability to provide appropriate wavelength-channel tracking and alignment in a WDM system and integrated device.
To date, devices have typically used what may be referred to as a "set and forget" scheme. In other words, existing devices have simply relied on the passband width of the WDM and/or transmitter(s) being large enough to tolerate any and all of the wavelength inaccuracies that may be present due to at least the reasons set forth above. In such a system, the wide WDM passband requires large channel spacing and also significantly limits the number of channels that can be effectively used in that communication system.